How to download a file on the three different platforms?

Hello,

I want to create a simple test app to download a file from the web into my local storage on Android, iOS and UWP. I have searched the web and looked at the online examples but I just can't get it to work. Can someone please show the steps to do this in a new Xamarin forms project, either as a Shared Project or as PCL? I am using Visual Studio 2017.

Thanks!